I once tried to block up a car with cinder blocks. Very bad idea, as the block cannot withstand a point load. As I slowly let the car down onto the blocks, the blocks just crumbled away.
Jack stands are cheap, so I have a couple extra I stick under the car whenever I get under it. After I have the car up on stands, I give the car a good shaking and shoving to make sure it is stable.
I keep a fire extinguisher at hand.
Most people remember to use eye protection. Few think of using ear protection. The problem is, you won't know you've damaged your ears until years later (the doctor told me that my hearing loss now is because of what I did 20 years ago). Whenever I use power tools now, I pop in either the cheap (but effective) foam plugs you can get at the drugstore, or a good pair of earmuffs you can get wherever they sell guns.
I've had a couple of gasoline accidents, one where the hose came off and sprayed gas everywhere. I stopped everything I was doing, and just left until the gas evaporated away (open the doors and windows by hand).
I installed one of those battery shutoff switches on the battery terminals. Whenever I work on the car, I just turn the knob and disconnect the battery.
If the engine is a new build or hasn't run for a long time, roll it outside before firing it up. That way, if it burns, it won't take your house with it.